PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = "E. Guinness, 2000-09-01; E. Guinness, 2001-05-28" RECORD_TYPE = STREAM OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = VL1 INSTRUMENT_ID = LR1 OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "LABELED RELEASE MODULE" INSTRUMENT_TYPE = "BETA DETECTOR" INSTRUMENT_DESC = " Instrument Overview =================== The Labeled Release (LR) instrument was part of a package of three biology experiments on the Viking Landers. All were assembled in clean rooms and heat-sterilized with the entire spacecraft to prevent the transport of terrestrial organisms to Mars [KLEIN1974]. The two Viking Landers had identical LR instruments that used two solid-state beta detectors to measure the release of radioactive gas from samples of martian soil that had been inoculated with an aqueous nutrient solution. The instrument also contained sensors to measure head-end assembly and detector temperatures. Heaters were also included in the LR instrument for the detectors and test chambers. A number of published papers describe the characteristics and performance of the LR instrument. The scientific rationale and early design of the instrument are described in LEVIN1972 and a detailed description of the flight module is given in LEVIN&STRAAT1976A. A summary of the information from these papers is given here as a high-level description of the LR instrument and its operation. Science Objectives ================== The major scientific objectives of the Viking Lander biology investigation were to detect the plausibility and/or presence of life on Mars. In particular, the LR experiment was designed to detect microbial life in the martian soil. The LR experiment tested for heterotrophic metabolism by monitoring the release of radioactive gases from a soil sample inoculated with carbon-14 labeled organic substrates. The experiment also was designed to analyze heat sterilized control samples for comparison to the unheated samples. Several assumptions were used in the design and operation of the LR experiment. These assumptions included the following: A) life on Mars was carbon based; B) one or more of the nutrient compounds would be metabolized by possible microbial life; and C) one end product of metabolism would be a carbon based gas. LR Detailed Description ======================= The LR instrument contained four incubation test cells mounted on a carousel. Each test cell could be rotated and sealed beneath a head-end assembly that contained a heater, plumbing terminals for nutrient delivery and gas removal, and a tube leading to the beta detectors. Gas and nutrient moving through the plumbing system were controlled by eight miniaturized solenoid valves. The test cell headspace (i.e, the volume of test cell above the sample) was connected to the two solid-state beta detectors through a tube bent at several spots. The connecting tube was bent to prevent the detectors from seeing the radioactivity of the liquid nutrient, and to prevent radioactive particles from reaching the detectors from the test cell, permitting only gas to pass from the test cell to the detector. The instrument also had two temperature sensors. One sensor measured the temperature of the detectors and the other measured the temperature at the head-end assembly. For a schematic and photographs of the LR instrument see LEVIN&STRAAT1976A. The LR nutrient was stored in a sealed glass ampoule within a reservoir. The reservoir, in turn, was connected to the test cells by the instrument plumbing system. The ampoule containing the nutrient was broken by a mechanical striker driven by high pressure helium shortly after the spacecraft landed on the surface of Mars. Low pressure helium was then bubbled through the nutrient in the reservoir for several hours in order to degas the nutrient. At the start of an analysis cycle high pressure helium was used to route a portion of the nutrient into a sealed test cell. Testing prior to launch indicated that 0.115 cc +/- 8% of nutrient was delivered during each injection. The pressure in the test cell headspace was kept above Mars ambient atmospheric pressure with helium to prevent boiling of the nutrient. The total pressure at the start of an analysis cycle was about 9200 Pa (92 millibar). At the end of an analysis cycle, radioactive gas was purged from the test cell through the plumbing system. The LR instrument had two identical solid-state beta detectors to monitor the evolved radioactive gas from the soil sample. There were two detectors as a contingency against failure of one detector. The detectors continuously counted gaseous radioactivity as it evolved from the test cell. The instrument could be commanded to count with either one or both detectors. Detectors were referred to as the right and left channels. The Viking Landers were powered by radioisotope thermoelectric generators (RTGs), which provided background radiation to the detectors. This background signal was determined by monitoring the detectors for a period after the sample was delivered to the test cell, but before nutrient injection. The test cell heaters were used in a number of ways. Samples could be heated in the test cell prior to nutrient injections to provide sterilized control samples. During an analysis cycle the test cell was heated to prevent the temperature from falling below 9-10 degree C during the martian night to prevent freezing. The heaters were also used at the end of an analysis cycle to dry the sample. The LR instrument shared common support services with the other biology instruments. This included the sample delivery system [KLEIN1974]. Martian soil samples were collected by the Viking Lander surface sampler arm [MOOREETAL1987; CROUCH1977]. Soil from the sampler arm was dumped into a hopper on top of the lander. The hopper, which was open to the atmosphere but shielded from sunlight, contained a sieve that only allowed particles less than 2 mm in size to enter the instrument test cells. The sieved samples entered a distribution assembly that automatically delivered measured volumes of soil to each biology experiment [KLEIN1974]. The LR instrument typically received 0.5 cc of soil. The Biology common support services and the LR instrument held enough nutrient and helium to conduct two injections on each of four soil samples. Testing prior to launch showed that after proper drying of a soil sample and purging of gases from a test cell that the cell could be used a second time by adding more soil and nutrient. One test cell on Viking Lander 2 was used a second time. Nutrient Description ==================== An integral part of the LR experiment was the nutrient used in sample incubation. Considerable effort went into selecting, preparing, and testing the nutrients for the Label Release experiment [LEVIN&STRAAT1976A]. Selection criteria were partially based on the assumption that life evolved similarly on Mars as on Earth. There were several criteria used in selecting compounds for the nutrient that included: A) Compounds that were likely to have been produced on Mars from its primordial atmosphere by Miller-Urey type reactions; B) Compounds that a wide variety of terrestrial microorganisms used in metabolism based on an extensive test program; and C) Compounds that were unlikely to have nonbiological reactions with martian soil. In the test program, each compound in the nutrient was tested to show that it produced a rapid response in a variety of terrestrial soils and in pure and mixed cultures of hundreds of types of organisms. The stability of the materials was also considered given that the nutrient was stored for about 2 years from the time it was prepared until it was used on Mars. The storage period included the nearly one-year cruise phase of the spacecraft on its journey to Mars. Although filter-sterilized prior to being sealed in its ampoule, nutrient material also had to undergo sterilization of the biology module (54 hr at 120 degree C) and the entire spacecraft (20 hr at 100 degree C). It was expected that some decomposition (<1%) of the nutrient would occur because of the long storage time and sterilization. Any radioactive carbon dioxide produced by the nutrient decomposition was removed by flushing the nutrient reservoir with helium before the first injection. A complete list of nutrient compounds is given in LEVIN&STRAAT1976A. The nutrient included sodium formate, calcium glycolate, glycine, D- and L-alanine, sodium D-lactate and sodium L-lactate. The concentrations of each nutrient component were dilute in case any particular compound might be toxic to possible martian organisms. Each compound in the nutrient was labeled with a precise amount of radioactive carbon-14 [LEVIN&STRAAT1976A]. The nutrient solution was unbuffered so that it would not alter the natural pH in the martian soil. Operation and Sampling Modes ============================ The basic analysis cycle for the LR experiment began with the delivery of a martian soil sample to a test cell and moistening of the sample with an aqueous solution of carbon-14 labeled organic media. A second nutrient injection was typically done about 7-8 Mars days after the first injection. The head-space above the sample was monitored continuously for evolved radioactive gas as evidence for metabolism. Viking Lander 1 completed four cycles, whereas Viking Lander 2 completed five cycles for a total of nine cycles between the two landers. VL2 cycle 5 was performed with a previously used test cell using a stored portion of the cycle 4 soil sample. Seven of the cycles received two nutrient injections. VL1 cycle 3 received three nutrient injections, whereas VL2 cycle 5 received only one nutrient injection. Three of the analysis cycles were control cycles(VL1 cycle 2, and VL2 cycles 2 and 4) where the soil sample was heated for several hours before nutrient injection. The control cycle for Viking Lander 1 was heated to 160 degree C. After receiving highly attenuated results from the Viking Lander 1 control cycle, the two Viking Lander 2 control cycles were modified to heat the sample to 50 degree C in order to improve the discrimination between biological and chemical responses. Radioactivity was accumulated in 16-minute bins during most of an analysis cycle, except for several hours around the time of nutrient injection when bin intervals of 2- to 4-minutes were used. Background levels were determined by measuring radiation penetrating the detector cell prior to nutrient injection. The head-end and detector temperatures were measured every 16 minutes throughout an analysis cycle. At the end of the cycle, the radioactive gas was removed by purging and the soil was dried by brief heating to prevent evaporation upon opening the cell. Gases and water vapor were vented to the martian atmosphere. A fresh test cell was then rotated beneath the head-end assembly and a three-hour cleanup was accomplished by heating both the test cells and the detectors during continuous helium purging. After cooling, trapped nutrient was vented from the system. The LR instrument was fairly automated with preprogrammed sequences. However, commands could be sent from the ground to change the preprogrammed sequences to perform nutrient injections, to select active or control sequences, to change the temperature of the control sample, to select a fresh soil sample, and to initiate or terminate an analysis cycle. Commands also could be sent to conduct single or double channel counting. Principal Investigator ====================== The principal investigator for the Viking Lander LR experiment was Dr. Gilbert V. Levin of Biospherics, Inc. Much of the work on the nutrient was directed by Co-Investigator Dr. Patricia Ann Straat." END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "CROUCH1977" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "KLEIN1974" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "LEVIN1972" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "LEVIN&STRAAT1976A" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "MOOREETAL1987" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END